Recently, in the dye image forming process using a silver halide light-sensitive color photographic material, high temperature development and simplification of processing procedures have been attempted to shorten developing time. To reduce developing time in high temperature color development, it is crucially important to increase developing rate. Developing rate is greatly affected by the following two factors: the type of a silver halide light-sensitive material and the type of a color developer. As to the former factor, the shape and size of silver halide grains, as well as the composition of a silver halide light-sensitive emulsion greatly influence developing rate. As to the latter factor, development rate is affected by the conditions of a color developer, in particular, the type of a development inhibitor. It is known that silver chloride grains are developed at a significantly high rate under specific conditions. In the case of a silver halide light-sensitive color photographic material with silver halide emulsion layers containing silver halide grains each consisting substantially of silver chloride (hereinafter referred to as a silver chloride light-sensitive color photographic material), development rate can be increased considerably as compared with the case of conventional silver chlorobromide light-sensitive materials. Accordingly, the use of silver chloride light-sensitive materials leads to such advantages as a shortened processing time, the employment of small-sized automatic developing machine, a decreased replenishing amount and a less possibility of environmental pollution. Because of these merits, a silver chloride light-sensitive material is expected to be used advantageously for the work in a miniature laboratory that has been attracting a great deal of attention. Various studies were made on the practical use of such silver chloride light-sensitive color photographic material.
It was revealed that, however, in a silver chloride light-sensitive photographic material, sensitivity and gradation were greatly affected by the intensity of exposure, as compared with the case of light-sensitive materials containing silver halide grains with other composition.
Meanwhile, the dependence of sensitivity on exposure intensity at a given amount of exposure is well-known in the art. This phenomenon is not a serious obstacle to successful photographing, since it can be prevented, for instance, by varying the amount of exposure according to anticipated changes in sensitivity.
On the other hand, the dependence of gradation on exposure intensity is a serious problem from a practical viewpoint. Desired gradation is varied according to purpose, and each light-sensitive material is so designed that it will obtain prescribed gradation. When a light-sensitive material is exposed to light, exposure conditions, for instance, the intensity of exposure may vary according to the brightness of objects (in the case of photographing) and the density of an original film (in the case of printing). The use of a light sensitive material which is unable to avoid influence of exposure intensity on gradation leads to the formation of an image with gradation falling outside the prescribed range. In such image, gradation is so hard that details in lower or higher density portions can not be reproduced, or so soft that the image has a dull appearance as a whole.
Light-sensitive photographic printing papers vary in size, generally from E size to full size. Usually, users print some scenes to small-sized printing papers, and then select favorable ones for enlargement. Enlargement is performed by using the same original film as employed in printing to a small-sized printing paper, without a significant increase in the intensity of light source. Therefore, enlargement is inevitably accompanied by a lowering of exposure intensity. When a light-sensitive material which is unable to avoid influence of exposure intensity on gradation is employed for enlargement, gradation is adversely affected by a lowering in exposure intensity, impairing significantly the quality of an image.
As stated above, it is possible to prevent sensitivity from being affected by exposure intensity by using improved exposure equipment. However, as for the influence of exposure intensity on gradation, the use of improved equipment cannot be an effective countermeasure. This problem should be solved by the improvement of a light-sensitive material itself.
The use of iridium compounds was already proposed as a method for making gradation less dependent on exposure intensity. However, studies made by the inventors revealed that, by the conventional technique as to the use of iridium compounds, it was impossible to prevent gradation from being affected by exposure intensity without causing adverse effects on other photographic properties.
The first example of such conventional technique is disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter abbreviated as Japanese Patent O.P.I. Publication) Nos. 147142/1981, 23146/1986, 211142/1983, 97648/1986 and 7042/1987. According to this technique, an iridium compound is added in advance to a mother liquor prior to nucleation or growth of silver halide grains; or is added with a rush to a reactor during the growth of silver halide grains.
However, in this method, a large amount of an iridium compound is needed to make gradation less dependent on exposure intensity, which inevitably causes a significant lowering in sensitivity. Since a silver chloride emulsion inherently has a lower sensitivity, the use of a large amount of an iridium compound significantly impair the quality of a light-sensitive material.
The second example is described in Japanese Patent Examined Publication No. 23248/1982, Japanese Patent O.P.I. Publication Nos. 106424/1976, 205930/1986, 260137/1987, 49752/1988 and 83719/1988. According to this technique, a soluble halide solution containing an iridium compound is added at the time of forming silver halide grains.
However, by the above technique, it is impossible to attain the object of the invention; that is, to obtain a silver halide emulsion containing a silver halide grain having a high silver chloride content, which is improved in sensitivity and the dependence of gradation on exposure intensity, and suited to rapid processing. In the method described in Japanese Patent Examined Publication No. 23248/1982 and Japanese Patent O.P.I, Publication No. 106424/1976, silver halide grains are formed by the single-jet method or the reverse-jet method. Grains obtained by this method have a broader grain size distribution, and hence, gradation is affected greatly by developing temperature and developing time. The method described in Japanese Patent O.P.I. Publication Nos. 205930/1986, 260137/1987, 49752/1988 and 83719/1988 was schemed out for an emulsion having a high silver bromide content. The application of this method to an emulsion having a high silver chloride content leads to a significant lowering in sensitivity.
The inventors carried on studies, and found that, as compared with a silver halide emulsion having a high silver bromide content, a silver halide emulsion having a high silver chloride content requires a smaller amount of an iridium compound than that needed in the case of adding with a rush to a reactor or adding in advance to a mother liquor.
In Japanese Patent O.P.I. Publication Nos. 275256/1987, 275259/1987, 287250/1987, 11941/1987, 40154/1988, 259654/1987, 261349/1987, 304253/1987, 6941/1988, 26837/1988, 26838/1988 and 26840/1988, the addition of a smaller amount of an iridium compound to a silver halide having a high silver chloride content is described. However, the studies by the inventors revealed that an emulsion obtained in this way had a sufficient sensitivity, but was considerably poor in the linearity of gradation. The "linearity" of gradation as referred to herein means that the gradation of a lower density portion of an image is identical with that of a higher density portion. When such linearity is impaired, details in a lower or higher density portion cannot be reproduced at all. In the case of an emulsion obtained by the above method, since the gradation of a lower density portion is soft, while that of a higher density portion is hard, details in the higher density portion (shadow part) cannot be reproduced.
The inventors made extensive studies to obtain a silver halide emulsion having a high silver chloride content, which is improved in sensitivity, the dependence of gradation on exposure intensity, as well as the linearity of gradation. As a result, the inventors have found that the above object can be attained by a silver halide emulsion having a silver chloride content of not less than 90 mol%, which is obtained by forming silver halide grains in the presence of a water-soluble iridium compound and a nitrogen-containing heterocyclic compound forming a sparingly soluble salt with a silver ion; and by controlling the addition of said iridium compound to a reactor where said silver halide grains are formed in such a way that said iridium compound is added in an amount substantially proportional to the total surface area of the grains in the reactor.
The use of a nitrogen-containing heterocyclic compound in forming a silver halide grain is a well-known technique, and described, for example, in Japanese Patent Examined Publication Nos. 23248/1982, 35440/1987, 6942/1989, 38930/1988. However, none of these publications contains a description suggesting that the use of such compound leads to improvement in gradation linearity and other effects as attained by the present invention. It should also be noted that these effects are produced most satisfactorily in a silver halide emulsion having a silver chloride content of not less than 90 mol%. Satisfactory results cannot be obtained when the silver chloride content is smaller than 90 mol%.